As chip size continues to be reduced and signals are clocked at ever increasing frequencies, high-speed, high-frequency signals being driven between chips via traces on a printed circuit boards (“PCB's”) may be subject to detrimental impedance effects. One manifestation of these impedance effects is unwanted reflections due to impedance mismatches (i.e., impedance discontinuities between a signal trace and a component, for example, that may impair the performance and reliability of a circuit). The high-speed, high-frequency signals may also be prone to cross-talk and electromagnetic interference (“EMI”).
One way of verifying the impedance characteristics of a particular component on a PCB is to observe signal reflections from the component when high-speed test signals are launched onto the associated circuit board. Sample test areas are therefore incorporated into circuit board designs to facilitate connection to impedance measurement tools. Such test structures are used to verify behavior of the PCB itself, as well as various attached circuit packages, sockets, and connectors.
The industry-standard tool used for circuit board impedance measurements is the Time Domain Reflectometer (TDR). In setting up for actual circuit board impedance measurements, the TDR probing approach is of great concern. Impedance matching between the TDR connection and the board is critical. Any impedance discontinuity in this area will inevitably appear as an inaccurate TDR reading.
SMA (subminiature series A) connectors are sometimes used to couple TDR equipment to the circuit board for testing. SMA connectors are advantageous because they are inexpensive, possess high mechanical strength, and are easy to probe. However, this type of connector installation is usually less than satisfactory, since it may contribute to an impedance discontinuity between the probe and the circuit board at high frequencies.
To help reduce such discontinuities, the width of a trace within a PCB may be changed. Since the size of the signal trace affects its characteristic impedance, the impedance of the trace can be decreased by increasing the trace surface width. However, a wide trace may use too much PCB area (real estate) in certain implementations. Thus, there is a need to reduce impedance discontinuities in PCBs while also avoiding the use of overly wide traces.
The present invention provides circuit structures and methods that can be applied to test high-speed signals on circuit boards and components for high-speed signal performance characterization purposes. Such circuit structures and methods will minimize impedance discontinuities between high-speed components and the circuit board.